Girton Birdwatch - October 2010
Swifts, swallows and house martins are all leaving at this time of year, with the swift one of the earliest to go. Few can be seen after the middle of August, although there were 12-13 outside our window on the 29th presumably 'northern' birds on their way south. A few days later a similarly sized group of House martins were stocking up on food over the Rec, while down the lane over 30 swallows were resting on the telephone wires. It's always a slightly sad time of year with so many birds leaving our shores, although we know they'll be back in the spring, and we have the winter migrants to look forward to.
When talking about absences a much more worrying concern is the fate of the kestrel. This falcon is the bird of prey that everyone will have seen, it being an especially familiar sight as it hovers above our motorway verges. Like all birds of prey it is protected by law throughout the year but, unlike many of the others, for many years it was not thought to be in much need of this protection. Farmers recognised that the kestrel did a good job in controlling mice, rat and vole populations, not to mention harmful insects, and even the more enlightened game-keepers seemed prepared to overlook the loss of the occasional game chick given the kestrel's value as a destroyer of pests. One of my old bird books suggests that: 'Partly because it does not have to face persecution, and partly because it can adapt to many different kinds of country, the kestrel has become by far the commonest of Britain's day-flying birds of prey'. Moreover, because they have quite large broods, breed in their first year, and have always been prepared to use railway sidings and industrial wasteland, there were always safe populations even in the most intensively farmed parts of England. They were affected, like most birds of prey by the pesticide era of 1960-63, but since then have always managed to keep their numbers up; until now.
The latest Breeding Bird Survey records a 20% drop in the kestrel population between 1995 and 2008, and a further fall of 36% between 2008 and 2009. This is extremely worrying, especially as other birds of prey such as buzzards, red kites and hobbies have experienced increases in their numbers over the same period. No one seems to be completely sure why this decline in kestrels is occurring. Some people have suggested that the correlative increase in wind farms may be a factor, but others have pointed out that the kestrel's manoeuvrability and small size makes this unlikely as a major influence, and that their larger and higher flying relatives seem at more risk in this respect. A more convincing explanation would appear to be the vulnerability of their food supplies. Buzzards, red kites and hobbies have very different diets to kestrels. Buzzards and kites feed mainly on carrion, including roadkill, and there's plenty of that about. I remember when one could drive from Cambridge to Penzance without, for example, seeing a single badger carcass. Nowadays, 24 hour motorway and other high-speed driving create abundant carrion, as do rabbit population explosions and the accompanying myxomatosis outbreaks. The kestrel relies heavily on a ready availability of voles and other small mammals and should their numbers be threatened there will be a knock-on effect on kestrels. Mammal numbers rely upon the quality of their habitat and so, once again, intensive farming can be bad news for both prey and predators.
Fortunately we still have hunting kestrels around the village and a resident pair on the Rec. I used to think that they relied upon their acute eye-sight to identify and catch their prey which, to a considerable extent they do, but recently David Attenborough pointed out that some species of bird can see over a wider colour spectrum than ourselves and how useful it is for a kestrel to be able to detect ultra-violet light. Voles mark their tracks through the grass with squirts of urine, which reflect ultra-violet light. This, in turn, is detectable by a hunting kestrel.
It is still possible to see out-going birds such as whinchat, redstart and osprey. In the Spring, and despite my fears, wheatear were spotted on the guided bus line between here and Oakington, so a late straggler on its way south is still a possibility. There was a cattle egret on the Cam Washes recently.